An electric field communication refers to a communication having a communication scheme in which a transmitter induces an electric field and forms the electric field in a dielectric material and a receiver then detects the electric field, so as to enable transmission and reception of data. Among such an electric field communication, especially a communication employing a human body as a dielectric material is called a human body communication.
In general, the human body communication has high noise in a band of several MHz due to user interference, etc., and has noise distributed over bands of several scores of MHz. FIG. 1 shows the frequency characteristics of human body communication channels as described above. In the human body communication scheme as described above, the transmission efficiency is maximized by applying a modulation scheme suitable for characteristics of a channel for communication. Therefore, in the human body communication, transmitted data should be carried by carriers of a band capable of avoiding the band of several scores of MHz in which human noise is concentrated. However, in the human body communication, when the carriers are too high, elements radiated from a human body increase. Therefore, the frequency band of a signal transmitted through a human body usually has a band within several scores of MHz, in consideration of the increase of the radiated elements and the noise characteristics.
Further, in the human body communication, it is also important to maximize the efficiency of the transmission band, because it is usual that the human noise has an energy level higher than that of temperature noise. Therefore, the central frequency f0 should be located within scores of MHz as shown in FIG. 1, and a transmission with a maximum band efficiency is an important object to be achieved in the human body communication.
Meanwhile, in the human body communication, data transmission is possible through an electric field formed around a human body using the dielectric characteristic of the human body and is performed after an intuitive selection of a terminal by a user. Therefore, the human body communication can provide users with intuitive convenience. In order to maximize the intuitive convenience of a user in data transmission, it is necessary to transmit all data within a short time during which a human body makes a non-contact or proximity contact with an information terminal device, which requires the performance of high speed transmission. In the case of non-contact performance as described above, as the distance from a user's body increases, the magnitude of the electric field decreases at a ratio of 1/r2, which implies that the magnitude of a received signal rapidly decreases as user's body moves farther away. Accordingly, in order to compensate for signal attenuation due to the human body, the reception sensitivity of communication has been improved by various methods, such as methods using optical sensors, magnetic sensors, etc. or methods using electrodes for improving ground coupling. The limitation in the reception sensitivity as described above is limited to the case of communication through human contact, and makes the signal processing difficult in the case of a low signal level of a receiver. Further, since it is difficult to determine a human body proximity state or non-contact state, or an ultra-proximity state between devices, it is difficult to select a channel and a communication scheme proper for each state. Further, in the case of human body non-contact communication, the determination of whether to perform a communication is not made based on whether there is a human body contact, and only a proximity of a human body itself starts an operation of a communication system. Therefore, these points should be taken into consideration in the design of a human body communication system.